The dye-sensitized solar cell (DSSC) has been studied by observing charge transfer from an organic terthiophene dye into a CdS nanoparticle. Using NMR and UV-Vis, we find characteristics of dye aggregation and a concomitant reduction in the electron transfer efficiency as measured by ultrafast transient absorption (TA) spectroscopy. Specifically, the NMR and UV-Vis spectra of the dye molecules indicate aggregates are readily formed at high surface loading, or roughly a 20:1 dye:nanoparticle ratio. Upon analysis by TA spectroscopy the same samples show a dominant S<sub>1</sub> state quenching process separate from the expected intersystem crossing and electron transfer (ET) S<sub>1</sub> quenching pathways. We propose that the dominant process is concentration-quenching because it only appears at high surface coverage where aggregates are detected spectroscopically; at lower surface coverage (ratios of dye:nanoparticle of roughly 1:1) the ET mechanism is the dominant pathway for S<sub>1</sub> reduction and the parasitic concentration-quenching pathway is not observed. We therefore suggest that planar oligothiophene dyes should be modified to frustrate packing on the surface in an effort to avoid concentration quenching losses, or that dye loading be considered when creating a DSSC from planar dye molecules.
Classical molecular dynamics (MD) simulations are also presented to corroborate the experimental picture described above. These simulations show that dyes aggregate in a variety of orientations, and that dye molecules are stabilized by these aggregation events even in the presence of explicit solvent. The ability of the dye molecules to pack more densely than is found experimentally shows that the surface of the CdS nanoparticle is likely undersaturated. In this situation, dye molecules can be either uniformly distributed around the surface of the nanoparticle, or they can be concentrated in islands on certain crystallographic faces, leaving other faces unoccupied. The experimental signs of aggregation support the latter.
University of Minnesota Ph.D. dissertation. September 2013. Major: Chemistry. Advisor: Wayne L. Gladfelter. 1 computer file (PDF); ix, 177 pages, appendices A-C.
Vatassery, Rajan Nayar.
Computational and experimental studies of dye sensitized solar cells.
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